Long before the Spaniards came to the Philippines, settlements already existed along the coves (locally known as the looc) in the northwestern part of Luzon. This region, then known as Samtoy (from sao mi ditoy or our language) was a progressive trading post familiar to the ancient Japanese, Chinese and Malay traders and renowned for its gold mines. These settlements, called the Ylocos, from Bangui (Ilocos Norte) in the north to Namacpacan (Luna, La Union) in the south were discovered during the expedition led by Juan de Salcedo, grandson of the Spanish conquistador Miguel Lopez de Legazpi, on June 13, 1572.

Salcedo established his headquarters in a settlement along the Mestizo River, and made it capital of his encomienda for services directly rendered to Spain. Salcedo’s territory included the provinces of Ilocos Norte, Ilocos Sur, Abra and La Union. This capital, later known as Vigan, became the seat of the Archdiocese of Nueva Segovia in 1758, after it was transferred from Lallo, Cagayan Valley. Vigan was also called “Ciudad Fernandina” in honor of Prince Ferdinand, the son of King Philip of Spain.

The Spaniards’ effort to Christianize this whole region was never completely successful owing to the abusive conduct of the Augustinian friars including the maltreatment done by government and military personnel which led to several uprisings by the locals (the Ylocanos). On February 2, 1818, a Royal Decree was signed splitting the Ylocanos into two provinces: Ilocos Sur and Ilocos Norte and also annexed Lepanto and Amburayan in Mountain Province (Cordilleras). Act 2683 passed in March 1917 by the Philippine Legislature defined the present geographical boundaries of Ilocos Sur.